In one commonly utilized form of a filter element, the filter element defines a longitudinal axis and first and second axial ends of the filter element, with the filter element including a central core, and a length of fluted filter media wound about the central core, with the flutes of the media oriented substantially longitudinal, to thereby provide for filtration of a flow of fluid passing axially through the filter element. Such filter elements are typically installed within a housing, or duct, in such a manner that the filter element can periodically be removed for cleaning or replacement with a fresh filter element. A seal between the filter element and the housing is typically provided, in such a manner that none of the fluid flowing through the housing can bypass the filter element, to thereby ensure that all fluid passing through the housing must enter one axial end of the filter element and exit from the opposite axial end of the filter element.
Where possible, filter elements of the type described above are typically formed into a right circular cylindrical shape, with the filter media being wound about a round central core. In some applications, however, it is necessary to form filter elements into non-cylindrical shapes, to fit space constraints of the particular application. For example, in a so-called “race track shaped” element, the cross-sectional shape of the filter element is race track shaped, having a pair of curved (in some embodiments, semi-circular, ends joined by a pair of straight segments). In such race track shaped filter elements, and in filter elements having other non-circular cross-sectional shapes, such as oval or rectangular, for example, the length of fluted filter material is wound about a non-circular central core. Such non-circular central cores may have cross-sections that are substantially rectangular in shape.
In one prior approach to providing a race track shaped filter element, as disclosed in U.S. Pat. Nos. 7,001,450 B2; 6,746,518 B2 and 6,547,857 B2, to Gieseke et al., fluted filter media is coiled about a rectangular shaped center board. The center board includes a corrugated region, which purportedly helps to hold the fluted media in place during winding of the filter element.
As is well known in the art, and stated in the patents to Gieseke, fluted filter media typically exhibits an inherent shape memory which can cause the media to bow outwardly, radially, away from the center board. In a filter element having a cylindrical central core, this inherent curvature memory is typically not problematic, in that the curvature of the media will not interfere with, and may actually assist in, pulling the media down onto the core, or onto a previously wound layer of the media. For cores in race track shaped elements, however, such as the center board used in the Gieseke patent, the inherent tendency of the filter media to bow outward may make it difficult to create a secure, tight, seal between the first layer of the media and the center board.
According to Gieseke, the corrugations of the center board alleviate this problem because the fluted media is better able to mate with and engage with the corrugations of the center board than with a flat surface. In practice, however, the corrugated section of the center board of Gieseke may, in fact, make it more difficult to achieve the desired fit between the first layer of fluid media and the outer surface of a non-cylindrical core, by interfering with the ability of the media to move freely while being pulled by a winding tensioning force across the surface of the core during the winding operation. In addition, fluted filter media, of the type typically used in such filter elements, is fabricated from layers of paper-like material by processes which are not amenable to accurate control of dimensional tolerances, with the result being that, in actual practice, the fluted media may not fit well into the corrugations of the center board, thereby causing an additional tendency of the first layer of media to be pushed outward away from the surface of the center board.
As a further difficulty, the corrugations of Gieseke must be closely matched to a given flute profile, thereby making it difficult to utilize different flute spacing and shapes with a given center board, which in turn can drive up inventory and manufacturing costs. Because filter elements of this type are often disposable, so that they may be readily replaced at appropriate intervals, additional manufacturing complexity and cost, resulting in increase replacement cost of the element, is highly undesirable and can be a significant detriment to successful competition in the marketplace.
Because the central core, in a wound filter element, is essentially a non-operating part of the completed filter, it is desirable that the central core be fabricated at minimal cost, utilizing as little material as possible, and have a configuration which can be readily manufactured in a form that is also readily disposable or incinerable. It is also desirable, in some applications, that the central core be fabricated in a manner which is as light in weight as possible. The center board of the Gieseke patent does not meet these requirements. Although the center board of Gieseke includes a number of holes extending through the thickness of the center board, the center board of Gieseke includes considerable surplus material. This condition is exacerbated by the inclusion of the corrugated section.
The center board of Gieseke also discloses a cutout, at one end of the center board, to be held by a spindle during winding of the filter element. The configuration of the cutout of Gieseke is an inefficient drive mechanism, in that the winding torque from the spindle is apparently applied only to the sidewalls of the cutout, which are located closely adjacent to the winding axis and essentially formed by the thickness of the center board. Such an inefficient drive mechanism may make it difficult to apply sufficient winding torque, without damaging the center board, to maintain a winding tension in the filter media which is high enough to pull the media into intimate contact with the outer surface of the center board or previously wound layers of media.
Another prior approach to forming a race track shaped filter element is shown in U.S. Pat. Nos. 7,008,467 B2 and 6,966,940 B2, to Krisko et al. Krisko uses a core construction including a race track shaped non-cylindrical, imperforate, wall member, having opposing walls defining an open volume within the imperforate non-cylindrical member. In cross section, the non-cylindrical member has the appearance, generally, of a flattened cylindrical-shaped tube having flat opposing side walls joined at opposite ends by curved walls, surrounding the open volume. The opposing walls and ends of the non-cylindrical member of the core construction of Krisko are imperforate. The core construction of Krisko further includes structural moldings and plugs located within the open volume, for providing strength, and axially closing off the open volume to ensure that fluid cannot leak through the open volume inside of the non-cylindrical member.
The central core construction of Krisko is even more complex than the center board of Geiseke, as described above. The core construction of Krisko also appears to include a substantial volume of excess material, making such a core construction undesirably expensive to produce and heavier than necessary. The core construction of Krisko also results in a significant amount of extra material which must be disposed of or incinerated when the filter element is replaced.
It is desirable, therefore, to provide an improved method and apparatus for manufacturing a fluid filter element having filter media wound about a non-cylindrical central core, in a manner which overcomes one or more of the problems discussed above, and/or providing improved utility over the prior art.